1. Field of the Invention
This invention relates to a power transmission system and, more particularly, to a planetary gear transmission having dual input shafts which are summed to develop the output power and maintain constant speed output for variable speed inputs.
2. Description of the Prior Art
Planetary gear systems, sometimes referred to as differentials, are commonly used to accomplish combination or division of power flow between input and output drive shafts. A basic differential gear system consists of a central sun gear, a plurality of planet gears mounted on planet arms (the carrier) and rotating around the sun gear, and a ring gear surrounding all the other gears and engaging the planets. In such an arrangement, input and output drive shafts, usually coaxial and some of which may be concentric, are coupled respectively to the sun gear, the planetary gear carrier, and the ring gear. In such a system, power may be transmitted through the differential transmission in six different ways, each arrangement producing distinct speed and torque relationships among the three separate shafts. However, power ratios among the respective shafts are uniquely the same for all arrangements.
A common application of a differential gear assembly is the conventional rear axle gear unit in an automotive vehicle. This allows the drive shaft to turn the drive wheels at different speeds when the vehicle is going around a curve. In such an arrangement, the ring gear is coupled to the planetary gear carrier with two sun gears coupled to the respective sections of the split rear axle.
More complex and specialized planetary gearing systems have been developed for various special applications. For example, the Burton et al U.S. Pat. No. 2,581,320 discloses a gearing system to couple two engines to two counter-rotating propellers in an aircraft. Other systems coupling multiple power inputs to a common output are disclosed in U.S. Pat. Nos. 2,823,558 of Semar et al, 3,507,113 of Herrmann et al, 3,861,484 of Joslin, 3,127,791 of Roe, 3,151,502 of Kron et al and 3,971,266 of Inakura et al. In the arrangements of Roe and of Inakura et al, each input shaft is enabled to transmit power independently to the output.
So called free-floating planetary transmissions are the subject of U.S. Pat. Nos. 4,132,131 of DeBruyne and 3,540,311 of Chillson, the former disclosing, like Burton et al, an arrangement to drive counter-rotating output shafts from a plurality of inputs.
The arrangement of U.S. Pat. No. 3,479,909 of Fritsch includes multiple power paths from a single power source to a single gear output, as does the arrangement of U.S. Pat. No. 3,864,987 of Vigneri.
U.S. Pat. No. 3,307,426 of Whitaker discloses a complex transmission arrangement for coupling to a reversible turbine engine for starting an aircraft engine with various takeoffs for operating accessories. This system incorporates a constant speed drive for controlling the accessory at constant speed over variations in the engine speed range.
In a particular application to which the present invention is directed, it is desired to drive an aircraft generator at a constant speed of 12000 RPM to develop 400 hertz power at 75 kw capacity from a turbine having a nominal 2/1 variation in operating speed. Conventional design approaches to solving this problem utilize a split power path in which one of the paths includes a hydraulically controlled, continuously variable transmission (CVT) for providing speed control. Utilizing a split power path in this manner reduces the power requirements imposed on the CVT. However, such an arrangement when used in combination with a mechanical traction CVT and a conventional planetary gear transmission results in unacceptably high rotational speeds for the planet carrier and planetary gears. Accordingly, some arrangement is needed which is capable of combining the split power paths, driven at varying rates of speed and in reverse directions to each other, to develop a constant output speed. The input power over the two separate input paths is summed together over the speed ranges encountered without excessive planet gear and carrier speeds.